Conventional tunnel freezers comprise an elongate tunnel. Product to be frozen, for example food is carried through the elongate tunnel on the top of an endless conveyor which typically comprises stainless steel mesh. The product is frozen by contact with cold nitrogen which is introduced as liquid through spray bars at the product outlet end of the elongate tunnel. The nitrogen passes through the elongate tunnel counter current to the flow of product and gradually evaporates. It is then exhausted to atmosphere through an exhaust duct adjacent the product inlet.
In order to maintain good heat transfer between the cold gaseous nitrogen and the product, turbulence inducing fans are provided which are situated adjacent the roof of the tunnel between the spray bars and the exhaust duct. These turbulence inducing fans typically suck cold nitrogen upwardly towards the roof and blow it out radially. Typically, heat is transferred from the product in the immediate area of the spray bars at a rate of around 120 w/m.sup.2 /.degree. K. whilst it is around 40 w/m.sup.2 /.degree. K. in the vicinity of the turbulence inducing fans.
GB-A-1 494 104 discloses a tunnel freezer comprising an elongate tunnel, a conveyor extending through said elongate tunnel and at least one turbulence inducing fan for bringing cold air into contact with product to be frozen. The fan is disposed above the conveyor and an intake cowl is positioned above the conveyor to facilitate the passage of cold air from the conveyor to the intake of the turbulence inducing fan. Ducts are provided to guide the cold air from the turbulence inducing fan, over an indirect heat-exchanger, to the space below the conveyor. The cold air passes upwardly through the conveyor and is recycled to the turbulence inducing fan. No heat-transfer rates are quoted although Applicant's experiments indicate that it is less than Applicant's existing freezers.
The aim of at least preferred embodiments of the present invention is to increase the rate of heat transfer in the vicinity of the turbulence inducing fans.
According to the present invention there is provided a tunnel freezer comprising an elongate tunnel, a conveyor extending through said elongate tunnel and at least one turbulence inducing fan for bringing cryogenic vapour into contact with product to be frozen when said tunnel freezer is in use, characterized in that said turbulence inducing fan is disposed beneath said conveyor and is provided with an intake cowl which is positioned below said conveyor to facilitate the passage of cryogenic vapour from said conveyor to the intake of said turbulence inducing fan, means to duct cryogenic vapour from said turbulence inducing fan to the space above of said conveyor, and means to promote the formation in said space of at least one vortex which will rotate about an axis which is generally parallel to the axis of said elongate tunnel.
Preferably, the intake cowl extends beyond either edge of the conveyor.
Advantageously, the means to duct cryogenic vapour from said turbulence inducing fan to the space above said conveyor comprises a duct defined between said intake cowl and an internal wall of said elongate tunnel.
If desired the cryogenic vapour may be conducted to the space above the conveyor via a single duct to one side of the conveyor. In this case, the upper part of the elongate tunnel is preferably shaped to enhance the formation of the required vortex. For this purpose the top of the elongate tunnel may be arcuate. However, it is more preferred to provide an arrangement inducing means to direct cryogenic vapour from said turbulence inducing fan to the space above said conveyor comprising two ducts each of which is disposed on a respective side of said elongate tunnel and is defined between said intake cowl and an internal wall of said elongate tunnel.
Preferably, an internal wall of said elongate tunnel opposite said turbulence inducing fan is provided with a baffle to promote the formation of two counter rotating vortices. The baffle may conveniently take, in cross-section, the form of a cusp.
Preferably, said turbulence inducing fan is mounted on a shaft which extends through the wall of said elongate tunnel and is rotatable by a motor.
Advantageously, said elongate tunnel is provided with a multiplicity of turbulence inducing fans which may optionally be driven by a common motor.
Preferably, at least one deflector is provided to direct cold gas into said duct. Advantageously, said deflector comprises a pair of generally triangular diverter plates which are joined adjacent the base of said turbulence inducing fan and extend towards said intake cowl.
Advantageously, the diverter plates are positioned so that, in use, substantially equal volumes of cryogenic vapour are deflected to either side of said elongate tunnel.
Preferably, said diverter plates are inclined towards said intake cowl and away from said turbulence inducing fan.
Advantageously, said diverter plates are joined by a filler plate and the inside of said deflector is optionally filled with insulation.
Preferably, the interior of said elongate tunnel has, in cross-section, eight sides comprising a bottom, a first inclined surface sloping upwards from one edge of said bottom, a second inclined surface sloping upwardly from the other edge of said bottom, a first side wall extending upwardly from said first inclined surface, a second side wall extending upwardly from said second inclined surface, a third inclined surface extending from said first side wall to one edge of a top and a fourth inclined surface extending from said second side wall to the other edge of said top.
Advantageously, an upper portion of said freezer tunnel comprising said first side wall, said second side wall, said top and said third and fourth inclined surfaces can be raised relative to the remainder of said elongate tunnel to facilitate access to said conveyor.
For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings, in which: